2,645 research outputs found
Topological interactions between ring polymers: Implications for chromatin loops
Chromatin looping is a major epigenetic regulatory mechanism in higher
eukaryotes. Besides its role in transcriptional regulation, chromatin loops
have been proposed to play a pivotal role in the segregation of entire
chromosomes. The detailed topological and entropic forces between loops still
remain elusive. Here, we quantitatively determine the potential of mean force
between the centers of mass of two ring polymers, i.e. loops. We find that the
transition from a linear to a ring polymer induces a strong increase in the
entropic repulsion between these two polymers. On top, topological interactions
such as the non-catenation constraint further reduce the number of accessible
conformations of close-by ring polymers by about 50%, resulting in an
additional effective repulsion. Furthermore, the transition from linear to ring
polymers displays changes in the conformational and structural properties of
the system. In fact, ring polymers adopt a markedly more ordered and aligned
state than linear ones. The forces and accompanying changes in shape and
alignment between ring polymers suggest an important regulatory function of
such a topology in biopolymers. We conjecture that dynamic loop formation in
chromatin might act as a versatile control mechanism regulating and maintaining
different local states of compaction and order.Comment: 12 pages, 11 figures. The article has been accepted by The Journal Of
Chemical Physics. After it is published, it will be found at
http://jcp.aip.or
Surveys of the earth's resources and environment by satellites
The potential and promise of observing the earth from the vantage point of space is discussed. The systematic surveying of processes and phenomena occurring on the surface of the earth by Landsat 1 and Nimbus 5 is considered to be useful in the following areas: assessment of water resources; mineral and petroleum exploration; land use planning; crop, forest, and rangeland inventory; assessment of flood, earthquake, and other environmental hazards; monitoring coastal processes; environmental effects of industrial effluents and of air pollution; mapping the distribution and types of ice covering the earth's polar caps and global soil moisture distributions
Chaotic Orbits in Thermal-Equilibrium Beams: Existence and Dynamical Implications
Phase mixing of chaotic orbits exponentially distributes these orbits through
their accessible phase space. This phenomenon, commonly called ``chaotic
mixing'', stands in marked contrast to phase mixing of regular orbits which
proceeds as a power law in time. It is operationally irreversible; hence, its
associated e-folding time scale sets a condition on any process envisioned for
emittance compensation. A key question is whether beams can support chaotic
orbits, and if so, under what conditions? We numerically investigate the
parameter space of three-dimensional thermal-equilibrium beams with space
charge, confined by linear external focusing forces, to determine whether the
associated potentials support chaotic orbits. We find that a large subset of
the parameter space does support chaos and, in turn, chaotic mixing. Details
and implications are enumerated.Comment: 39 pages, including 14 figure
Development of advanced techniques for rotorcraft state estimation and parameter identification
An integrated methodology for rotorcraft system identification consists of rotorcraft mathematical modeling, three distinct data processing steps, and a technique for designing inputs to improve the identifiability of the data. These elements are as follows: (1) a Kalman filter smoother algorithm which estimates states and sensor errors from error corrupted data. Gust time histories and statistics may also be estimated; (2) a model structure estimation algorithm for isolating a model which adequately explains the data; (3) a maximum likelihood algorithm for estimating the parameters and estimates for the variance of these estimates; and (4) an input design algorithm, based on a maximum likelihood approach, which provides inputs to improve the accuracy of parameter estimates. Each step is discussed with examples to both flight and simulated data cases
Development of longitudinal handling qualities criteria for large advanced supersonic aircraft
Longitudinal handling qualities criteria in terms of airplane response characteristics were developed. The criteria cover high speed cruise maneuvering, landing approach, and stall recovery. Data substantiating the study results are reported
What does a binary black hole merger look like?
We present a method of calculating the strong-field gravitational lensing
caused by many analytic and numerical spacetimes. We use this procedure to
calculate the distortion caused by isolated black holes and by numerically
evolved black hole binaries. We produce both demonstrative images illustrating
details of the spatial distortion and realistic images of collections of stars
taking both lensing amplification and redshift into account. On large scales
the lensing from inspiraling binaries resembles that of single black holes, but
on small scales the resulting images show complex and in some cases
self-similar structure across different angular scales.Comment: 10 pages, 12 figures. Supplementary images and movies can be found at
http://www.black-holes.org/the-science-numerical-relativity/numerical-relativity/gravitational-lensin
- …